Here's the rundown, this is ran through the actual console and recorded with a capture card, there's no cheaitng involved either to get unobtainable cards (I'm looking at YOU, BEUD cheaters!), nor there's card duplication used, this was all through patience and pain:
Your first (or second if you didn't beat Labyrinth Mage on the Seto 2nd sidequest) guy to fight is Sebek, this guy can be a pushover.... MOSTLY, there's a huge exception, if you give him a chance he'll equip his metalzoa, and he can power up his regular Zoa (which is already field boosted!!) through fusion of two Yami cards, for a dark energy. That gives him a ridiculous ATK of 3600, so if you dont have enough good equips or a MBD, you're basically screwed. I kill him in a children's card game by just use of equips and using Yami to my favor. Big deal. But please, don't give him a chance to set up magics OR make him stack up monsters, he's got Shadow Spell and Spell Binding Circle, BOTH of which are extremely harmful to your monsters' ATK (-1000, -500 resp).
For Neku you're gonna have to try a little harder than that... he's got Zoa AND Skull Knight, while you can get a Skull Knight yourself from Low Meadow Mage, don't play chances here, he's got those two and... actually he's not really THAT scary, but don't let him get too safe.
For Heishin... oh boy, you, DO, need Meteor B. Dragon!! AND PUT IT ON SUN! His Gate Guardian will go down, and while you can live without it, remember that you'd need to feed quite the amount of steroids into whatever you want to power up to beat Mr Gate Guardian, one megamorph into twin headed thunder dragon Should do it, but I don't have it and therefore that strat sucks (jk, that's kinda viable but don't depend on a lucky draw, this is a gauntlet, remember)
I don't recall heishin using that much spells, but they're most likely equips, and Gate Guardian is one of those equip friendly cards, that you never get to use! You can see I just wasted a ton of cards just to get good draws. As for your filler cards, they should be powerful enough to deal additional damage to Heishin if your MBD did not kill him and send him to like, the shadow realm or something. Interestingly, he has the 5 pieces of exodia in his deck, so please beat him quick.
Okay so you got all this down? Well get ready for this one because here's where it all goes downhill.
Seto 3rd is by far, the HARDEST duel in the entire game, because of one monster that he has that, again, you cannot acquire legally outside of Japan. This one being the Blue Eyes Ultimate Dragon, and this card wil ruin your attempts, sporting a memorable 4500 ATK points that he never switches into defense (The AI will never do it if the monsters' base ATK is 3000 or more), and there's also the fact that these dickheads can read your face down monsters starting from Heishin, there's nothing you can do but hope you can kill that monster, you might be thinking, but Middy, Widespread Ruin and Raigekis right? You can use the latter reliable as your allstop signal, but Widespread Ruin? He has three copies of Harpie's Feather Duster, hell he might even have four due to morph cards, which basically transform into whichever card the AI needs because they cheat. He's got you pinned, hopefully after the first BEUD he will quit summonning, or at least pray he doesn't because he has THREE OF THOSE, Oh and he also has Shadow Spell, Raigeki, and all in all painful cards that will make sure you hate yourself by the end if you lose. Your best bet is either a two/three equipped MBD (bright castle x 3 or megamorph x 2, etc.) JUST so your own monster doesn't die. Two hits from that three headed beat will kill you, so dont hesitate and dump whatever useless crap haunts your hand. You CANNOT AFFORD MISTAKES basically.
Beat Seto 3rd? Good because it gets easier here, kinda. Meet Darknite, despite being the big bad he can be a pushover, in fact he had 3 trap cards in this run that tbqh i dont know what the fuck they were. nor I want to find out -- The strat here is to pummel him quick and neat, his strongest guy is barely Meteor Black Dragon, Blue Eyes White Dragon, however he's got Megamorph so dont even try to stall! He's got Reverse Trap and since you're usually running off of equips he will completely wreck ANY chances at a victory if you're not running MBD. Raigeki if you have to. Also Crush Card.
Well guess what, he's salty and wants one more game, sure. Now you'd think his deck is the same, but it isn't, because butts,. Instead, now he goes for an all out offensive, he has NO magic cards, only raw monster power, don't fuck around, Blue Eyes Ultimate Dragon and Gate Guardian are once again some of the cards he can puke out, which thankfully are not as common as some of his other draws, he can also Meteor Black Dragon, but again, don't hesitate, this bastard I swore cheated his way out regardless once and I lost cause of it.